Can I ask what the difference for you is between neutering a male and a female? Not all dogs will stop things like humping after being neutered. Depending how long ago he was neutered, it may eventually stop as sometimes it can take a little time. My question though would be, why is he exhibiting boredom behaviour in your opinion? It's imperative to ensure that a dog is mentally and physically stimulated or they will firstly be unhappy and secondly engage in undesirable behaviours, such as the one you mentioned or destructive behaviours. If you think he is bored then you need to provide more things that are going to stimulate him. Even if a dog has a lot of space, he needs to go on walks to wear him out, allow him bonding time with you, allow him to sniff and experience new things, meet new dogs and people and some say satisfy his migrating instinct. He will also need lots of things to chew to satisfy his instinct to chew and possibly things like kongs to mentally stimulate him. Teaching him commands and tricks are also good things for wearing him out mentally.
Obviously I'm not insinuating that you do not do any or all or these things, but these are my suggestions of what is important for a dog and if you believe he is exhibiting boredom behaviour then a good place to start would be introducing or upping the amount of time spent on these things.